Diffusible hydrogen reproducibility

Figure 8.7 Frames (23 by 35 A) of an STM movie taken at 65 K at close to a complete monolayer of hydrogen adatoms at Pd(l 11) showing vacancy diffusion. The images (b) and (c) show the aggregation of two nearest neighbour vacancies, which has the appearance of a three lobed object due to the rapid diffusion of one H atom next to the vacancy dimer. (Reproduced from Ref. 24).

It is, therefore, noteworthy that almost immediately upon Welsh and associates discovery of collision-induced absorption in hydrogen [128, 129, 420], Herzberg found the first direct evidence of the H2 molecule in the atmospheres of the outer planets [181, 182], He was able to reproduce in the laboratory the unidentified diffuse feature at 827.0 nm observed by Kuiper in the spectra of Uranus and Neptune, using an 80 m path of hydrogen at 100 atmospheres pressure and a temperature of 78 K. The feature is the S3(0) line of the 3 — 0 collision-induced rotovibrational band of the H2 molecule [182]. 

Any hydrogenated candidate for the unknown IR emission must exhibit sufficient stability to the harsh conditions of the interstellar medium, including bombardment by particles and ultraviolet and gamma radiation. Equally important, for a species to be considered a viable candidate, its spectra must reproduce two important characteristics found in most diffuse medium spectra. These are (a) that the 1,300 cm-1 band be more intense than the 880 cm-1 band (Williams 1996), and (b) that the spacing between it and the next main emission feature match the canonical interstellar spacing of 300 cm-1 (Hudgins and Allamandola 1999). 

Fig. 15. (a) Intramolecular hydrogen bonds in urea crystal with displacement ellipsoids at 50% probability, (b) Static deformation density obtained from the multipolar analysis of the experimental data corrected for the thermal diffuse scattering. Theoretical deformation density obtained using (c) the Hartree-Fock method (d) the DFT method by generalized gradient approximation (contours at 0.0675 eA-3) (reproduced with permission from Zavodnik et al. [69]). 

As the hot band transitions should exist in both of the v, band of hydrogen-bonded and physisorbed acetonitrile, we must get the accurate v, band without the effect by the hot band transitions. We have to determine the analytical functions for fundamental transitions of v/a and vJp and for hot band transitions upon the least-squares procedure. We assumed that the reorientational-vibrational relaxation processes of physisorbed acetonitrile could be described as a diffusion process like bulk liquid molecule, whose spectral density has a Lorentzian form. Accordingly it is supposed that the v,p band is reproduced as a sum of three Lorentzian curves of the v, p, v," p, and bands. In the previous study [9], it was assumed that the Vj a band has a Gaussian band shape and the hot band transitions could be ignored. In the present study we assumed that the v,a band over the range of PIP, =... 

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